PCI has 4 dedicated interrupt lines INTA# INTB# INTC# INTD# https://www.ics.uci.edu/~harris/ics216/pci/PCI_22.pdf

Yeah that is a great reference book on PCI spec.
Odd thing about GA-108, it does not have a paddle board to insert into ISA slot for the IRQ's/DMA's like many other PCI IDE controller cards so it could be more board/BIOS specific for full use.
The few PCI IDE controller cards I own have those paddles making them function in nearly all PCI motherboards by grabbing the IRQ's and DMA's off the ISA slot. Looking at Stasons or TH99 it says INTD# is default for Primary IDE controller, and INTC# for secondary for GA-108. Some motherboards assign INT# based on the slot, slot1 is A#, 2 is B#, etc. If the board has 5 PCI slots then 2 share a INT# iirc. Some boards like the Asus P6NP5 have INTA# on all PCI slots and the manual states: "thus all PCI cards must be set to this value" (pg42 or rev1.41 manual) and in BIOS are default set to AUTO.
The Tyan S1366 is similar to your card (based on pictures) using the CMD PCI 640b chip and has only jumpers to set INTA# (for Primary IDE channel) and INTB# (for secondary IDE channel) or No PCI INT# but it also has a ISA paddle.
Am amazed it actually see's the HD on a 486 PCI board , would expect that card to be for a Pentium like early Socket 4 or 5 without onboard controller I/O.
for shits-n-grins linked to Luckybob post about getting same card: Re: What retro activity did you get up to today?
below is pics of the Tyan S1366 PCI Multi I/O card for comparison, the ribbon cable has an ISA paddle for proper IRQ/DMA for the Floppy, Serial, Parallel, etc...
Just rambling and not sure